Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227, issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. A Spinal Cord Stimulation (SCS) system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver, electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. The electrodes, which reside on a distal end of the electrode lead, are typically implanted along the dura of the spinal cord, and the IPG or RF transmitter generates electrical pulses that are delivered through the electrodes to the nerve fibers within the spinal column. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode lead(s) exit the spinal column and generally attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG or RF receiver is implanted. Alternatively, the electrode lead may directly connect with the IPG or RF receiver.
SCS and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940, issued Mar. 7, 1972, entitled “Implantable Electronic Stimulator Electrode and Method,” which teaches timed sequenced electrical pulses to a plurality of electrodes. Another example, U.S. Pat. No. 3,724,467, issued Apr. 3, 1973, entitled “Electrode Implant for the Neuro-Stimulation of the Spinal Cord,” teaches an electrode implant for neuro-stimulation of the spinal cord. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes reside. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted and is controlled by an external controller.
U.S. Pat. No. 3,822,708, issued Sep. 9, 1974, entitled “Electrical Spinal Cord Stimulating Device and Method for Management of Pain,” teaches an SCS device with five aligned electrodes which are positioned longitudinally along the spinal cord. Current pulses applied to the electrodes block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes have a repetition rate of 5 to 200 pulses per second. A patient-operated switch allows the patient to change the electrodes that are activated (i.e., which electrodes receive the stimulation pulses) to stimulate a specific area of the spinal cord, as required, to better block the pain.
Regardless of the application, all implantable pulse generators are active devices requiring energy for operation. The energy is supplied by a power source that may be an implanted battery or an external power source. It is desirable for the implantable pulse generator to operate for extended periods of time with little intervention by the patient or caregiver. However, devices powered by primary (non-rechargeable) batteries have a finite lifetime before the device must be surgically removed and replaced. Frequent surgical replacement is not an acceptable alternative for many patients. If a battery is used as the energy source, it must have a large enough storage capacity to operate the device for a reasonable length of time. For low-power devices (less than 100 μW) such as cardiac pacemakers, a primary battery may operate for a reasonable length of time, often up to ten years. However, in many neural stimulation applications such as SCS, the power requirements are considerably greater due to higher stimulation rates, pulse widths, or stimulation thresholds.
Thus, one challenge with IPGs is keeping power usage to a minimum to conserve battery life. While increasing battery life may be achieved by extending the size of the battery, that runs counter to the goal of reducing the overall device size which is determined partly by battery size. Conservation of energy in an implantable, battery operated device is an important design goal to reduce the overall size of the device and to prolong the life of the battery, thus deferring surgery to replace the device.
An IPG often includes one or more output current sources that are configured to supply current to a load, such as tissue, associated with the IPG. The output current source may include a current digital to analog converter (DAC) configured to regulate the current that is delivered to the load. However, the DAC is often physically located in series with the load. Hence, any load current passes through the DAC as well, which results in a power loss. This power loss may result in a shortening of the battery life of the IPG. The power loss is directly proportional to the voltage drop across the DAC. Accordingly, there is a great need for an IPG having an output current source that includes a current DAC having a small voltage drop such that the power efficiency of the IPG is maximized.